1. Field of the Invention
The present invention relates to a normally open electromagnetic valve, and more particularly to a linear solenoid valve which can change a produced differential pressure according to a value of electric current which flow through a coil.
2. Description of Related Art
In general, a electromagnetic valve (solenoid valve) has a coil, a fixed core and a movable core and is configured such that a magnetic field is produced by electric current flowing through the coil, whereby the fixed core and the movable core are made to be attracted to each other, so that the valve can be opened and closed by means of the attraction force acting between the cores.
In the case of a normally open electromagnetic valve, a return spring is provided to cause a valve body to normally stay apart from a valve seat surface so as to keep a flow path open, and when the coil is energized, the valve body is pushed against the valve seat surface to close the flow path.
Incidentally, in a linear solenoid valve which can change a produced differential pressure according to a value of electric current flowing through the coil, the valve is devised such that a valve closing force that acts when a valve body is brought into abutment with a valve seat surface hardly varies due to a production error.
For example, as is described in JP-UM-A-60-24983 and JP-A-11-108230, a raised portion is provided at the center of a lower end face of a movable core so as to project towards a fixed core, while a recessed portion is provided on an upper surface of the fixed core so as to accommodate the raised portion. By adopting this construction, when a constant electric current flows trough a coil, with the raised portion left staying out of the recessed portion, magnetic flux is caused to converge to corner portions of the raised portion and corner portions of the recessed portion, the fixed core and the movable core strongly attract each other. On the other hand, just before the raised portion enters the recessed portion or after the raised portion has entered the recessed portion, since attraction force produced by magnetic flux which passes near the corner portions acts only in a radial direction of the fixed core and does not work as a force which attracts the movable core towards the fixed core, even though the positional relationship between the raised portion and the recessed portion, in other words, the distance between the fixed core and the movable core changes, the attraction force acting between the fixed core and the movable core is made to hardly change.
Then, in the positional relationship where the attraction force is difficult to change, by bringing a valve body into abutment with a valve seat surface, the closing force of the valve can be made stable irrespective of there being an error in assembling parts together, and a construction like this is generally adopted in linear solenoid valves.
In the electromagnetic valve, however, since the valve body is repeatedly brought into abutment with the valve seat surface, it is inevitable that wear or fatigue is produced on the valve body and the valve seat surface. Due to this, even though the electromagnetic valve works to its designed specifications while maintaining its designed properties at an initial stage of use, after the use for a long period of time, an area over which working fluid pushes up the valve body (referred to as a “pressure-receiving area”) tends to decrease, and this causes a problem that a relationship between the electric current that is caused to flow through the coil and the pressure of the working fluid that is used to open the valve body, that is, a produced differential pressure changes.